Electroluminescent display and method for making the same

ABSTRACT

A method for constructing an electroluminescent display and a novel display constructed thereby. In the method of the present invention, a bottom electrode having a planar sheet of conducting material is first deposited on a substrate. A mask is then deposited on the bottom electrode to define wells. For at least first and second ones of the wells defined by the mask, an electroluminescent material is dispensed into the wells. The electroluminescent material dispensed into the first well emits a different spectrum of light from the electroluminescent material dispensed into the second well. In one embodiment of the invention, an isolation layer is deposited over the electroluminescent material after the electroluminescent material has dried. A top electrode is then deposited over the isolation layer. In a display according to the present invention, a plurality of pixels are deposited on a bottom electrode and then a top electrode is provided for energizing the pixels. The plurality of pixels includes first and second pixels, the first pixel having a first mixture of at least two electroluminescent dyes and the second pixel having a second mixture of the electroluminescent dyes. The first mixture differs in the ratios of the electroluminescent dyes from the second mixture.

FIELD OF THE INVENTION

The present invention relates to display devices, and more particularly,to displays utilizing polymer-based electroluminescent devices.

BACKGROUND OF THE INVENTION

Polymer-based electroluminescent devices (PLEDs) have the potential forproviding inexpensive alternatives to semiconductor-based LEDs. PLEDsmay be fabricated by coating the appropriate surfaces with an organicpolymer, and hence, do not require the use of high cost fabricationsystems such as those utilized in the fabrication of semiconductordevices. A simple PLED may be constructed from an electroluminescentlayer sandwiched between an electron injection electrode and a holeinjection electrode. More complicated devices utilize electron and holetransport layers between the above mentioned electrodes and theelectroluminescent layer.

In spite of the lower construction costs inherent in PLEDs, multicolordisplays based on such devices are still quite costly to fabricate. Toconstruct a multicolor display, a patterned deposition of each of aplurality of electroluminescent compounds must be performed. In priorart fabrication systems, a series of masking operations is required toprotect the areas that are not to receive a particularelectroluminescent compound. The electroluminescent compound is thendeposited using vapor deposition, dipping, or spin casting. The mask isthen removed and the next mask constructed using conventionalphoto-resist techniques. Each masking operation increases thefabrication costs and reduces the device yield. Hence, it isadvantageous to avoid steps that require masking.

The range of colors that may be generated by a display depends on therange of intensities that can be generated by each color of PLED. Inconventional multicolor displays, each pixel of the picture is generatedby controlling the intensity of each of three sub-pixels, one for eachof the primary colors. If a particular electroluminescent dye has alimited dynamic range, the resulting pixel of the image will be limitedto less than the full range of colors.

Finally, it is difficult to generate inexpensive displays that display asingle fixed image of the type used in point of sale advertising. If aconventional display is used, the image must be stored in a memoryexternal to the display and the display operated in a mode in which thestored image is continuously scanned into the display. The pixels of thedisplay must be individually addressable which further increases thecost of the display.

Broadly, it is the object of the present invention to provide animproved display based on PLEDs.

It is a further object of the present invention to provide a displaythat may be more inexpensively fabricated than prior art displays.

It is a still further object of the present invention to provide animproved method for fabricating PLED displays.

These and other objects of the present invention will become apparent tothose skilled in the art from the following detailed description of theinvention and the accompanying drawings.

SUMMARY OF THE INVENTION

The present invention is a method for constructing an electroluminescentdisplay and a novel display constructed thereby. In the method of thepresent invention, a bottom electrode having a planar sheet ofconducting material is first deposited on a substrate. A mask is thendeposited on the bottom electrode to define wells. For at least firstand second ones of the wells defined by the mask, an electroluminescentmaterial is dispensed into the wells. The electroluminescent materialdispensed into the first well emits a different spectrum of light fromthe electroluminescent material dispensed into the second well. In oneembodiment of the invention, an isolation layer is deposited over theelectroluminescent material after the electroluminescent material hasdried. A top electrode is then deposited over the isolation layer. In adisplay according to the present invention, a plurality of pixels aredeposited on a bottom electrode and then a top electrode is provided forenergizing the pixels. The plurality of pixels includes first and secondpixels, the first pixel having a first mixture of at least twoelectroluminescent dyes and the second pixel having a second mixture ofthe electroluminescent dyes. The first mixture differs in the ratios ofthe electroluminescent dyes from the second mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a display according to the presentinvention.

FIG. 2 illustrates the fabrication of a display according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in terms of a PLED based displayin which each pixel is a four layer structure of the type shown inFIG. 1. However, it will be apparent to those skilled in the art thatfrom the following discussion that other pixel configurations may beused. Refer now to FIG. 1 which is a cross-sectional view of a display100 according to the present invention. Display 100 includes a anode 104which is transparent and through which the image is viewed. The anode ispreferably constructed from indium tin oxide deposited on a transparentsubstrate 101. Glass is typically used for substrate 101. Each pixelincludes an electroluminescent layer 108 which is covered by a electrontransport layer 106. Electron transport layer 106 improves theefficiency of injection of electrons from cathode 102 intoelectroluminescent layer 108 by providing a material that has an energyband between that of the electroluminescent layer material and that ofthe cathode. Cathode 102 is preferably constructed from a metal with alow work function such as Ca, a LiAl alloy, or Mg.

While the above described embodiment of a PLED utilizes a specificgeometry involving an electron transport layer, it should be emphasizedthat other geometries are possible. For example, a five layer structurein which a hole transport layer is sandwiched between the anode andelectroluminescent layer is also known to the art. Further, embodimentsin which the hole transport layer and/or electron transport layer areabsent are also known to the art. As will become apparent from thefollowing discussion, the method of the present invention may be used inconjunction with all of these arrangements.

If the pixels of the display are to be individually addressable, theelectrodes are typically patterned to form rows and columns. Forexample, bottom electrodes may be patterned in rows and the topelectrodes in columns. An individual pixel in the display is thenaddressed by connecting the corresponding row and column.

As noted above, a plurality of electroluminescent materials must be usedfor the various pixels to provide a multicolor display. The presentinvention makes use of the observation that there are electroluminescentmaterials that are soluble in solvents that are compatible withdispensing systems that can accurately dispense small amounts of thesolvents at precise locations. For example, the three primary colors maybe generated from polyflourenes dissolved in an appropriate solvent withdopants to provide the colors. These compounds may be dissolved inxylene. The color is determined by the dye used in the doping. Examplesof suitable electroluminescent materials are known to those skilled inthe art. The reader is referred to "Electroluminescence of doped organicthin films", Tang, et al., J. Applied Physics 55, pp 3610, 1989, whichis hereby incorporated by reference, for a more detailed list ofsuitable materials. For example, the Coumarins and DCM compoundsdisclosed therein provide emissions in the blue-green and orange-redportions of the spectrum, respectively.

To simplify the following discussion, a fabrication method based on adispensing system that is analogous to the mechanism used in inkjetprinters will be described. Other forms of dispensers will be discussedin more detail below. An inkjet printer dispenses small droplets byvaporizing part of the liquid and using the vaporized liquid to propelthe droplet toward the receiving surface. Resolutions of 300 dropletsper inch are commonly achieved with inexpensive printer mechanisms.

Refer now to FIG. 2 which illustrates the fabrication of a displayaccording to the present invention. The locations of the pixels aredefined by a single mask 131 which is deposited on top of the bottomelectrode 132 which may be the anode or the cathode shown in FIG. 1. Themask may be generated by conventional photo-resist masking techniques.Each pixel location consists of a well which is to receive a droplet ofelectroluminescent material. An exemplary well is shown at 133. Thedroplets 138 are dispensed into the wells by a dispenser 136 which moveswith respect to the display being fabricated on a rail 137. As notedabove, dispenser 136 includes a plurality of reservoirs 139, one foreach electroluminescent material to be dispensed. The electroluminescentmaterials are preferably electroluminescent dyes that are dissolved in acarrier liquid. In the embodiment shown in FIG. 2, three reservoirs areutilized. As the reservoirs pass over the wells, controller 135dispenses droplets into the well in accordance with a predeterminedpattern that is stored in controller 135. In essence, controller 135prints the pixel pattern on the masked bottom electrode. Mask 131prevents the droplets from running into one another.

After the droplets have dried, mask 131 is removed leaving the isolatedelectroluminescent pixels. The area between the pixels and over thepixels is filled with a electron or hole transport material tofacilitate the injection of charges into the electroluminescent layers.In a co-pending patent application (U.S. Ser. No. 08/813,962 filed Mar.3, 1997), the manner in which the hole and/or electron transport layeris constructed is discussed in detail. The reader is directed to thatpatent application which is hereby incorporated by reference for adetailed discussion of the relationship between the hole transport layerthickness and the height of the electroluminescent pixels. For thepurposes of the present discussion, it is sufficient to note that thethickness of the transport layer may be chosen such that the transportlayer also electrically isolates the anode and cathode in those regionsnot containing electroluminescent material.

Alternatively, the photoresist mask may be left in place and transportlayer applied over it. In this case the mask provides the electricalisolation between the individual pixels. This alternative has theadvantage of eliminating one processing step.

Finally, the top electrode is deposited. In embodiments in which thepixels are individually addressed, the cathode and anode are typicallyconstructed as strips running at right angles with respect to oneanother to form a plurality of rows and columns. Each pixel is thenaddressed by powering the row and column electrodes connected to thatpixel. In embodiments in which a fixed "picture" is displayed by thedevice, the cathode and anode can be constructed as uniform conductivesheets.

For advertising purposes, an electroluminescent display which generatesa fixed image analogous to a transparency that is back illuminated wouldbe advantageous if such a display can be provided inexpensively. Themethod of the present invention can be used to provide such a display.In such an embodiment, each pixel is constructed from a mixture of theelectroluminescent materials. The ratio of the materials is set pixel bypixel to provide the desired color for that pixel when the pixel ispowered. Such a display utilizes a continuous cathode and anode. In thiscase, a fourth "dye" or filler material is utilized to fill those pixellocations that are not to generate light. This filler material fills thewells in the mask thereby assuring that the top surface on which thetransport layer and top electrode is deposited is more nearly planar.Otherwise it is constructed in a manner identical to that describedabove. When a voltage is applied across the anode and cathode, thedevice generates the desired image.

While the above described embodiments utilized an inkjet printermechanism for dispensing the electroluminescent dyes, it will beapparent to those skilled in the art from the above discussion thatother dispensers may be utilized without departing from the teachings ofthe present invention. For example, dispensers based on vibratingnozzles are known to the art. In these devices, the liquid leaving thevibrating nozzle breaks up into small droplets which are deflected by anelectrostatic field. Similarly, dispensers based on micro-pipettes areknown in the art. Other ink dispensers such as used in pen plotters mayalso be utilized.

Likewise dispensers comprising an array of micro-pipettes forreplicating a pattern stored in a two dimensional array of wells areknown in the liquid handling arts utilized by biochemistry andmicrobiology. In this case, the pattern of pixels is stored in themaster array of wells and is replicated on each of the devices utilizingthe array of micro-pipettes. This technology is most applicable fordisplays having large pixels.

The above described embodiments of the present invention utilized wallsto confine the droplets of electroluminescent materials. However, itwill be apparent to those skilled in the art from this discussion thatother forms of containment can be utilized. For example, the surface onwhich the droplets are deposited can be arranged in a plurality ofhydrophilic or hydrophobic regions so that the droplets are confined bysurface tension.

The above described embodiments of the present invention have utilized"pixels" which are in the form of dots. However, other configurationscan be utilized. For example, display elements that are lines or morecomplex structures can also be utilized. Such complex elements can beused as sub-images or components of images.

Various modifications to the present invention will become apparent tothose skilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

What is claimed is:
 1. A method for constructing an electroluminescentdisplay comprising the steps of:depositing a bottom electrode comprisinga sheet of conducting material; depositing a mask on said bottomelectrode, said mask defining wells; and for at least first and secondones of said wells defined by said mask, dispensing electroluminescentmaterial into said wells, said electroluminescent material dispensedinto said first well emitting a different spectrum of light from saidelectroluminescent material dispensed into said second well.
 2. Themethod of claim 1 further comprising the steps ofallowing saidelectroluminescent material to dry; and depositing a top electrodecomprising a conducting material.
 3. The method of claim 2 furthercomprising the step of depositing an isolation layer over said driedelectroluminescent material before depositing said top electrode.
 4. Themethod of claim 3 wherein said isolation layer comprises a carriertransport material having an energy band intermediate between that ofsaid electoluminescent material and said material conducting material ofsaid top electrode.
 5. The method of claim 3 further comprising the stepof removing said mask prior to depositing said isolation layer.
 6. Themethod of claim 1 wherein a third one of said wells is filled with acompound that does not emit light.
 7. The method of claim 1 wherein saidfirst well receives a first mixture of three electroluminescent dyes andsaid second well receives a second mixture of said threeelectroluminescent dyes, said first mixture differing in the ratios ofsaid three electroluminescent dyes from said second mixture.
 8. Themethod of claim 1 wherein said step of dispensing comprises dispensingdroplets of electroluminescent dyes dissolved in a carrier liquid from adispenser having at first and second reservoirs containing first andsecond electroluminescent dyes, respectively.